This little effect was sudden and rapid, so rapid that Strether's sense of it was separate only for an instant from a sharp start of his own.

He too had within the minute taken in something, taken in that he knew the lady whose parasol, shifting as if to hide her face, made so fine a pink point in the shining scene.

It was too prodigious, a chance in a million, but, if he knew the lady, the gentleman, who still presented his back and kept off, the gentleman, the coatless hero of the idyll, who had responded to her start, was, to match the marvel, none other than Chad.
Chad and Madame de Vionnet were then like himself taking a day in the country--though it was as queer as fiction, as farce, that their country could happen to be exactly his; and she had been the first at recognition, the first to feel, across the water, the shock--for it appeared to come to that--of their wonderful accident.

Strether became aware, with this, of what was taking place--that her recognition had been even stranger for the pair in the boat, that her immediate impulse had been to control it, and that she was quickly and intensely debating with Chad the risk of betrayal.

He saw they would show nothing if they could feel sure he hadn't made them out; so that he had before him for a few seconds his own hesitation.